Axial compressive response of large-capacity helical and driven steel piles in cohesive soil

The axial compression performance of large-capacity helical piles is of significant interest because they can offer an efficient alternative to conventional piling systems in many applications such as in oil processing facilities, transmission towers, and industrial buildings. This paper presents the results of seven full-scale axial compression load tests conducted on 6.0 and 9.0 m large-capacity helical piles and a 6.0 m driven steel pile. The results are considered essential to qualify and quantify the performance characteristics of large-capacity helical piles in cohesive soils. The test piles were close-ended steel shafts with an outer diameter of 324 mm. The test helical piles were either single or double helix, with a helix diameter of 610 mm and interhelix spacing that varied between 1.5 and 4.5 times the helix diameter. The subsurface soil properties at the test site were determined using field and laboratory testing methods. The 6.0 m piles were tested 2 weeks after installation, while the 9.0 m piles were tested 9 months after installation. The load-settlement curves were presented to better understand the behaviour of test piles. An ultimate capacity criterion was proposed to estimate the ultimate load of large-capacity helical piles. The test helical piles developed ultimate resistances up to 1.2-1.8 times that of the driven pile. The load-transfer mechanisms of large-capacity helical piles were studied, and it was found that soil disturbance during pile installation had a significant effect on the pile failure mechanism regardless the value of the interhelix spacing to helix diameter ratio. The mobilized soil strength parameters were back-calculated and compared with the estimated intact soil strength parameters.